Manganese and Sulfate Removal from a Synthetic Mine Drainage Through Pilot Scale Bioreactor Batch Experiments

Mine drainage is a significant problem in the Appalachian Plateau due to elevated metal and solute concentrations. Most metals may be removed by oxidation/precipitation or natural buffering, but Mn is more difficult to remove due to its higher solubility. Some mine drainages in southeastern Kentucky have average sulfate and Mn concentrations exceeding 1,300 and 30 mg L⁻¹, respectively. Manganese does not readily form sulfidic minerals, and MnS precipitation following sulfate reduction has not proven to be a promising pathway for permanent Mn immobilization. Our study involved batch experiments with five different organic carbon sources in combination with five inorganic substrates to treat a simulated mine drainage with pH 6.2 and Mn and sulfate concentrations of 90 and 1,500 mg/L, respectively. The Mn removal capacity varied widely between treatment mixtures, from <10 to 100%. Sulfate removal showed a similar divergence, ranging from <10 to >80%. The most effective treatment was provided by the biosolids or wood mulch amendments in combination with the creek sediment. Sulfate reduction levels were not stoichiometrically matched with MnS formation, suggesting that the prevalent Mn removal mechanisms were sorption and precipitation as oxide, oxy-hydroxide and carbonate, rather than Mn-sulfide phases.     

Assessing Pilot-Scale Treatment Facilities with Steel Slag-Limestone Reactors to Remove Mn from Mine Drainage

Pilot-scale mine water treatment facilities were operated for over four years at the Ilwol mine, South Korea. A steel slag-limestone reactor (referred to as the slag reactor) was tested and a successive alkalinity producing system (SAPS) and a SAPS incorporating slag from a basic oxygen steelmaking furnace were compared. The SAPS decreased Mn from 23.3 to 7.4 mg L^?1 on average because the alkalinity generated led to saturation with rhodochrosite. Adding a slag reactor removed Mn down to levels of 0.002–1.8 mg L^?1 from influent Mn as high as 17.1 mg L^?1 with a residence time of 5–25 h. Mn-containing carbonates and oxides were precipitated, which was supported by the geochemical modelling and observed with scanning electron microscopy with energy dispersive spectroscopy. The increased alkalinity in the SAPS before the slag reactor helped remove Mn at a pH range of 8.0–8.3. Mn removal rates and Mn-standardized Mn removal rates in the slag reactor were 0.76 mg L^?1 h^?1 and 0.105 h^?1 in average, respectively. The passive treatment of Mn using an Fe-pretreatment and alkalinity-generation system, a slag-limestone reactor, and a wetland rather than a SAPS including slag, an oxidation-settling pond, and a wetland is suggested to consistently meet the effluent standards for Mn and pH.

     

Тhe Еffectiveness of Metal and Metalloid Sorption from Mining Influenced Waters by Natural and Modified Peat

This study investigated the sorption behaviour of natural (N peat) and HCl-acid-modified peat (HCl peat) for contaminants in water collected at a mine site in northern Finland. Batch sorption experiments were conducted at room temperature and at 5 °C. Characterization of the sorbents by FTIR and XPS revealed no substantial change in the peat’s functional groups due to the acid treatment. Generally, the N peat was a more efficient sorbent for the mine water, although the HCl peat exhibited better nickel uptake capacity (21 mg Ni/g) than the N peat (16 mg Ni/g) from synthetic water. This is attributed to the lower equilibrium pH in samples treated with the HCl peat as well as the water’s different chemical composition. At room temperature, the N peat removed As(V) (80%) and Ni (85%) at low dosage (1–2 g/L), whereas the HCl peat presented good removal of As(V) (80%) at low dosage (1 g/L) but did not achieve satisfactory removal of Ni, even at a higher dosage (4 g/L). The performance of both sorbents was significantly affected by contact time. Ni removal by N peat increased substantially with contact time whereas removals achieved by HCl peat increased slightly up to 60 min, but decreased significantly at 24 h. Unlike with HCl peat, the N peat was less efficient in the experiments conducted at 5 °C. Overall, for both sorbents, As(V) and Ni were the most efficiently removed contaminants from the mine water. HCl peat had slightly better settling properties, however, both products settled poorly, thus rendering the studied mixing and settling system unsuitable for the proposed application. Nevertheless, both peat products, and especially the N peat, exhibited high contaminant removal potential and could represent a cost-effective and sustainable option for mine water treatment.     

Field Trials of Low-cost Reactive Media for the Passive Treatment of Circum-neutral Metal Mine Drainage in Mid-Wales,UK

This paper addresses the ability of five low-cost reactive materials to remove Zn, Pb, and Cd from Fe-poor, circum-neutral pH metal mine water in Mid-Wales, UK. Compost, fly ash, waste shell material, iron ochre, and a mixture of blast furnace slag (BFS) and basic oxygen furnace slag (BOS) were used in a series of small-scale passive treatment cells to assess metal removal from mine drainage initially containing, on average, 23.5 mg/L Zn, 0.5 mg/L Pb, and 0.05 mg/L Cd. Trial treatment cells contained between 1.5 and 12 kg of reactive media, had a 15 min residence time, and treated a discharge of up to 1 L per minute. Fly ash from a peat-fired power station was found to be the most effective material for metal removal, with concentrations reduced to 0.02 mg/L Zn, 0.0069 mg/L Pb, and 0.0001 mg/L Cd from over 1,000 L of water (between 98.6 and 99.9% removal). The other materials initially achieved high levels of metal removal (between 75 and 99.9% Zn, Pb, and Cd removed); however, all of the materials were saturated with Zn after less than 200 L of water had been treated. Metal sorption ranged from 21.4 mg/g Zn for the peat fly ash to 0.0015 mg/g Cd for the compost and BOS/BFS slag. The results of the pilot-scale field trials can be scaled to demonstrate that a modest-sized fly ash treatment cell (2.6 × 2.6 × 1 m) in size would be sufficient to remove 90% of the total metal load (Pb, Zn, and Cd) from this 10 L/min mine water discharge for a 1 year period. Importantly this research demonstrates that passive treatment for metal mine drainage can comply with water quality directives but cannot be considered a ‘walk-away’ solution; it requires modest (potentially annual) maintenance.     

Lime Treatment of Mine Drainage at the Sarcheshmeh Porphyry Copper Mine,Iran

Laboratory and field treatment tests were performed to evaluate the effectiveness of lime treatment for mitigation of environmental effects of acid mine drainage (AMD) at the Sarcheshmeh porphyry copper mine. AMD associated with the rock waste dumps is contaminated with Al (>36,215 μg/L), Cd (>105 μg/L), Co (>522 μg/L), Cu (>53,250 μg/L), Mn (>42,365 μg/L), Ni (>629 μg/L), and Zn (>12,470 μg/L). The concentrations of other metals (Fe, Mo, Pb, and Se) are low or below detection limits (As, Cr, and Sb). Due to the very high Al and Mn content and the low concentration of Fe, a two-stage lime treatment method was chosen for the laboratory tests. In the first stage, the AMD was treated at four pH set points: 7.5, 8.9, 9, and 10. In the second stage, after removing the sludge at pH 9, treatment was continued at pH 10 and 11. The results indicated that a two-stage treatment method was not necessary because elements such as Al, Cu, Co, and Zn were easily treated at pH 7.5, while complete removal of Cd, Mn, and Ni only required a pH of 10. Increasing pH during the treatment process only caused a slight increase in Al. Field treatment tests support the laboratory results. Lime treatment of highly contaminated AMD from dump 11, using simple low density sludge pilot scale equipment, show that contaminant metals are treatable using this method. The mean treatment efficiency for contaminant metals was 99.4% for Al, % for Cd, 99.6% for Co, 99.7% for Cu, 98.5% for Mn, 99.7% for Ni, 99% for U, and 99.5% for Zn. The optimum pH for AMD treatment by lime was in the range of 9–10. The produced sludge in the treatment process was highly enriched in the contaminant metals, especially Cu (>7.34%), Al (>4.76%), Mn (>2.94%), and Zn (>1.25%). A correlation coefficient matrix indicates that the distribution pattern of the contaminant metals between soluble and precipitated phases is consistent with the hydrochemical behavior of the metals during the lime treatment process.     

The application of wood ash as a reagent in acid mine drainage treatment

The paper deals with a possible utilisation of wood ash as a reagent in treating acid mine drainage (AMD) from opencast mining of brown coal. Wood ash samples were obtained having combusted deciduous and coniferous tree wood in a household furnace. The dominant mineral phases in wood ash are calcite, quartz, lime and periclase. The used AMD is characteristic of high contents of sulphates, iron, manganese, heavy metals and low pH. The AMD treatment process included dosing of wood ash to adjust pH values about 8.3 (a dose of 0.5 g l⁻¹) or calcium hydroxide (a dose of 0.2 g l⁻¹) for comparison. The reaction time was 20 min. Dosing of wood ash in AMD resulted in an increase of pH in solution from 3.5 to 8.3, which caused the removal of metal ions mainly by precipitation, co-precipitation and adsorption. Comparing the application of Ca(OH)₂ in AMD treatment, at an almost identical pH value the concentrations fell in both cases for Fe, Mn, As, Co, Cu, Ni, Zn, Mg, Al and Mo. Applying wood ash the drop was even more distinct in Mn, Zn and Mg. The results of sedimentation tests in an Imhoff cone confirm that the settling capacities of sludge using wood ash are significantly better than when using calcium hydroxide in acid mine drainage treatment.     

Passive Treatment of Circumneutral Mine Drainage from the St. Louis Mine Tunnel,Rico CO: Part 2—Vertical Biotreatment Train Pilot Study

This is the second of three papers dealing with metal-bearing circumneutral mine drainage from the inactive Rico-Argentine mine site located at an elevation of ≈ 2740 m (9000 feet) in the San Juan mountain range in southwestern Colorado. This paper evaluates two years of mine drainage treatment using a passive system that included a vertical-flow engineered biotreatment cell. The collapsed St. Louis Tunnel (SLT) discharges circumneutral mine water from several sources that contains elevated concentrations of Cd, Cu, Fe, Mn, Zn. A demonstration-scale 114 L/min (30 gpm) gravity-flow passive treatment system was installed, consisting of a settling basin (utilizing coagulant addition to improve suspended solids settling efficiency), an anaerobic sulfate-reducing bioreactor, and an aeration cascade for effluent polishing. The treatment system generally met target treatment goals for Cd, Cu, Fe, and Pb. Nanophase ZnS in system effluent decreased the frequency of meeting total Zn project treatment goals. Unexpectedly high levels of Mn removal were observed in both the anaerobic bioreactor and the aeration cascade. Large seasonal variations in influent metals concentrations and pH present the greatest challenge in managing system performance.

     

基于生物阴极MFC的PRB中试系统处理AMD研究

为研究基于SRB生物阴极微生物电池(MFC)的固定化硫酸盐还原菌可渗透反应墙(PRB)对提高PRB处理酸性矿井废水(AMD)效果的影响,构建了中试系统,考察了进水不同p H值、重金属离子浓度及不同HRT对系统处理AMD效果的影响。结果表明:(1)MFC区有很好的调节p H功能,进水p H=3,仍能调节PRB生化反应区进水及系统出水稳定在6.5左右。(2)重金属离子的去除以MFC区为主,系统最终出水重金属离子去除率在95%左右;调节HRT=4~2 d,Fe~(2+),Cu~(2+),Zn~(2+),Ni~(2+)去除速率750~1 500 mg/(m~3·h),而Mn~(2+)去除速率略低为600~1 000 mg/(m~3·h)。(3)硫酸根的去除速率随碳源减少呈下降趋势;MFC外电压随碳源以及菌量的变化而显著变化。     

煤矿地下水库对含不同赋存形态有机物及重金属矿井水净化效果研究

以神东矿区大柳塔煤矿在用的3座地下水库为研究对象,采集了3个进水、4个出水及1个裂隙水水样,通过对采集水样中有机物及其重金属含量测试,分析地下水库岩体对水体中的污染物的去除效果。研究结果表明,大柳塔煤矿地下水库采空区岩体组分中,黏土矿物含量约占35%,有较强的吸附能力。大柳塔煤矿地下水库系统对矿井水有较好的净化效果,出水悬浮物含量低于182mg/L,去除率可达80%～93%,能去除大量的悬浮物|出水COD含量小于35mg/L,去除率可达38%～61%,能去除部分COD|出水TOC含量在7.37～13.28 mg/L之间,去除率为19.1%～46.4%|分析表明颗粒态有机物可随悬浮颗粒物沉降而得到去除,而可溶性有机物可通过黏土矿物的吸附而得到去除。数据显示,进水中Fe和Mn各有99%和84%以上赋存于悬浮颗粒物上,经地下水库处理后,矿井水中Fe的去除率可达68%～100%,Mn的去除率可达75%～99%,表明地下水库针对赋存于悬浮颗粒物上的重金属可起到一定的去除作用。     

Passive Treatment of Circumneutral Mine Drainage from the St. Louis Mine Tunnel,Rico, CO: Part 1—Case Study: Characteristics of the Mine Drainage

This publication is a case study of the seasonal variability of mine water drainage from the Saint Louis Tunnel (SLT) at the inactive Rico-Argentine mine site located in southwestern Colorado. It is an introductory paper for the two passive water treatment system technology evaluations contained in this issue. Mine water chemistry changes from baseflow to a snowmelt runoff event (SMRE) where snowmelt runoff follows preferential migration pathways to flush acidic weathering products from the upper mine workings to the SLT. Baseflow mine drainage is characterized as circumneutral, with Zn, Cd, Mn, and Ni concentrations primarily in the dissolved form. Dissolved Zn, Mn, Fe, and potentially Cd illustrate equilibrium with carbonate minerals. Total concentrations of Fe, Cu, Pb, and As are primarily in the suspended form and suggest sorption to Fe oxides. Mine water chemistry during the SMRE reflects mixing of circumneutral baseflow waters with more acidic waters flushing the upper mine workings. Geothermal activity provides for a consistently warm mine water discharge from the SLT. The two seasons that provide the most challenge to passive water treatment of SLT mine drainage are the SMRE period and the low flow stage of the Dolores River. Mine water flow and chemistry during SMRE are highly correlated with Dolores River flow and this site conceptual model was and will be used to assist in pilot project evaluation, water treatment system design, monitoring system design, a seasonal compliance approach, and water management.

     

Evaluation of Metal Attenuation from Mine Tailings in SE Spain (Sierra Almagrera): A Soil-Leaching Column Study

A laboratory study was undertaken using mine tailings and soil columns to evaluate some of the natural processes that can control the mobility of metals at Pb–Ag mine tailings impoundments. The effects of buffering, pH, and salinity were examined with tailings from the El Arteal deposit. Al, Ba, Cd, Cu, Fe, Mn, Ni, Pb, Sr, and Zn were mobilized when the tailings were leached. However, when the mine tailings were placed above alluvial soils, Al, Ba, Cd, Cu, Mn, Pb, and Zn were retained, although Fe and Sr clearly remained mobile. Most of the metal retention appears to be associated with the increase in pH caused by calcite dissolution. The sorption of some metals (Cu, Pb, and Zn) onto oxyhydroxides of Fe and Mn, sulphates, clay materials, and organic matter may also explain the removal of these metals from the leachate.     

赤泥复合颗粒去除Fe~(2+)、Mn~(2+)影响因素及吸附性能

为了探究赤泥复合颗粒处理重金属酸性矿山废水的最佳反应条件,本研究对复合颗粒处理酸性矿山废水中Fe~(2+)、Mn~(2+)的影响因素及竞争吸附特性进行了深入研究,结果表明:赤泥复合颗粒投加量3 g/L、吸附时间120 min、pH值为4.0时反应条件最佳,对Fe~(2+)、Mn~(2+)的去除率可达99.41%、94.27%;Fe~(2+)、Mn~(2+)共同存在时,Fe~(2+)、Mn~(2+)存在竞争吸附,其中Fe~(2+)优先被去除。赤泥复合颗粒是处理重金属酸性矿山废水的优良水处理功能材料。     

The Influence of Engineering Scale and Environmental Conditions on the Performance of Compost Bioreactors for the Remediation of Zinc in Mine Water Discharges

The effects of engineering scale on the performance of a compost-based system for the remediation of a discharge from an abandoned metal mine was investigated by simultaneous operation, under field conditions, of a laboratory-scale column and a pilot-scale system. The two systems contained identical reactive substrate, comprising limestone gravel, compost, wood chips and activated sludge from a municipal waste water treatment plant, and had an initial hydraulic residence time of approximately 19?h. The influent mine water contained around 2?C2.5?mg/L zinc and had a circumneutral pH. Clear differences in the performance of the systems were seen, demonstrating the importance of engineering scale in the remediation of zinc. The laboratory-scale column was most effective at removing zinc, with approximately 96?% of the influent zinc attenuated within the system, while the pilot scale system removed, on average, 84?% of the influent zinc. The poorer performance of the pilot-scale reactor may, in part, be due to preferential flow, as indicated by a greater reduction in hydraulic residence time than in the laboratory-scale system. Early indications are that temperature also plays an important role in the attenuation of zinc within such systems, possibly linked to reduced microbial activity during periods of low temperature. Despite an apparent decrease in sulphate concentration within both systems, it is unclear whether bacterial sulphate reduction is the dominant mechanism for metal removal or whether sorption processes prevail. Implications for full-scale design of these treatment systems are discussed.     

Evaluation of the Potential for Beneficial Use of Contaminated Water in a Flooded Mine Shaft in Butte,Montana

The mines of Butte, Montana include over 16,000 km of abandoned underground workings, most of which are now filled with water. The feasibility of using the flooded mine workings as a source of irrigation water was investigated. The geochemistry and stable isotopic composition of water produced during a 59 day pumping test of the flooded Belmont Mine workings are described. Although static water in the pumping well initially met proposed irrigation standards, the quality deteriorated during pumping as water from deeper in the mine complex was drawn into the well. Stable isotopes show that this lower-quality water was not sourced from the nearby Berkeley Pit lake, but most likely came from the mine shaft itself. At steady state, the water pumped to the surface had pH 5.5–6.0 with high concentrations (in mg/L) of dissolved SO₄ (1,600), Fe (160), Mn (19), Zn (15), and As (1.8). Despite substantial bicarbonate alkalinity (≈150 mg/L as CaCO₃), the water became strongly acidic after equilibration with air due to oxidation and hydrolysis of Fe²⁺. Benchtop experiments were performed to test different strategies for low-cost chemical treatment prior to irrigation. The most feasible alternative involved aeration (to remove large quantities of dissolved CO₂) prior to pH adjustment to >9 with lime or NaOH. Further work is needed to see if such treatment is economically viable compared to the cost of using municipal water. Another concern is whether irrigation of grass with high TDS, high sulfate water is sustainable. The mine water reached a steady-state temperature of 19°C during pumping, and therefore the possibility of using this water to help heat nearby buildings should also be explored.     

Application of coal fly ash to circumneutral mine waters for the removal of sulphates as gypsum and ettringite

South African power stations generate large amounts of highly alkaline fly ash (FA). This waste product has a serious impact on the environment. Acid mine drainage (AMD) is another environmental problem associated with mining. AMD has high heavy metal content in addition to high sulphate concentrations. Several studies have shown that 80–90% of sulphates can be removed when FA is co-disposed with AMD rich in Fe and Al. In South Africa, sources of contaminated mine waters, unlike AMD have circumneutral pH and much lower concentrations of Fe and Al, but rich in Ca and Mg. Treatment of such waters with FA resulted in no significant removal of sulphates when treated to pH less than 10. Subsequent treatment of circumneutral mine water to pH greater than 11 resulted in more than 60% sulphate removal. Treatment of circumneutral mine water to pH greater than 11 with FA followed by seeding with gypsum crystals and the addition of amorphous Al(OH)₃ resulted in removal of sulphate to levels below the Department of Water Affairs and Forestry (DWAF) water quality effluent limit (500 ppm).     

A Bench-scale Assessment of a Combined Passive System to Reduce Concentrations of Metals and Sulphate in Acid Mine Drainage

Abstract  Passive treatment of acid mine drainage (AMD) requires a combined strategy to minimize the effect of climatic variability on the treatment performance of the system. A vertical-flow combined passive treatment system was developed and evaluated in a bench-scale laboratory test for a 290-day period. The combined system consisted of four components with specific treatment functions: an oxidation/precipitation basin for excess iron removal; a peat biofilter for heavy metal sorption and the establishment of anoxic conditions; a bioreactor for alkalinity generation and sulphate reduction; and an anoxic limestone drain for alkalinity addition. The benchscale system was dosed with moderate strength synthetic AMD at a surface loading of 95 L/m²/d, and operated under continuous flow conditions. Removal efficiencies were 99.7%, 99.9%, 99.9%, 98.6%, 98.2%, and 99.9% for Fe, Al, Zn, Mn, Ni, and Cu, respectively, while Cd remained more mobile with a removal efficiency of 66.5%. Sulphate concentrations were reduced from 3030 mg/L to 814.9 mg/L and the acidic drainage was neutralized to an effluent pH of 7.2 and an alkalinity of 1353.6 mg/L (as CaCO₃).     

Improving Mine Water Quality by Low Density Sludge Storage in Flooded Underground Workings

This paper discusses the chemical and physical characteristics of low density sludge (LDS) and its interaction with mine water in a flooded German underground fluorite mine. The highly hydrous nature of the sludge (11.5–17 % solids), its rather low sedimentation rate, and its thixotropic viscosity were confirmed. The interaction of LDS and mine water was tested in the laboratory in batch experiments and modelled with PHREEQC. Mine water quality improved through contact with the LDS sludge: the total alkalinity and pH of the water increased and its iron concentration and total acidity decreased. Storage of sludge in a flooded mine could be a sustainable tool for both the handling of LDS and improvement of mine water quality, even when the LDS represents less than 1 % of the total mine water volume. No polymer flocculants from the LDS treatment plant were found in the discharged mine water.     

Neutralizing Coal Mine Effluent with Limestone to Decrease Metals and Sulphate Concentrations

Abstract.   This paper describes pilot scale tests of a novel process for the neutralisation of acidic mine water. Leachate from a waste coal dump was neutralised with limestone, and iron, aluminium, and sulphate were removed. Specific aspects studied were: the process configuration; the rates of iron oxidation, limestone neutralisation, and gypsum crystallisation; the chemical composition of the effluents before and after treatment; the efficiency of limestone utilisation; and the sludge solids content. The acidity was decreased from 12,000 to 300 mg/L (as CaCO₃), sulphate from 15,000 to 2,600 mg/L, iron from 5,000 to 10 mg/L, aluminium from 100 to 5 mg/L, while the pH increased from 2.2 to 7.0. Reaction times of 2.0 and 4.5 h were required under continuous and batch operations respectively for the removal of 4 g/L Fe (II). The iron oxidation rate was found to be a function of the Fe (II), hydroxide, oxygen, and suspended solids (SS) concentrations. The optimum SS concentration for iron oxidation in a fluidised-bed reactor was 190 g/L. Up-flow velocity had no influence on the rate of iron oxidation in the range 5 to 45 m/h. Sludge with a high solids content of 55% (m/v) was produced. This is high compared to the typical 20% achieved with the high density sludge process using lime. It was determined that neutralisation costs could be reduced significantly with an integrated iron oxidation and limestone neutralisation process because limestone is less expensive than lime, and a high-solids-content sludge is produced. Full scale implementation followed this study.     

International Trials of Vertical Flow Reactors for Coal Mine Water Treatment

Vertical flow reactors (VFRs) were tested at coal mine sites in New Zealand, South Korea, and the USA. The objective was to evaluate the iron removal efficiency and iron removal mechanisms during field trials at low pH and circumneutral pH, and to evaluate the potential use of VFRs as stand-alone systems or in combination with other passive treatment technologies. Total iron and manganese removal efficiencies at circumneutral pH (6–8) often exceeded 90%, with effluent concentrations less than 1 mg/L. This is attributed to both homogeneous and heterogenous Fe(II) oxidation and filtration of the precipitated ferrihydrite. Iron removal efficiencies at moderately acidic conditions (pH 3–4.5) averaged close to 40%, with an average 71.0% removal in one of the trials after iron removal capacity was stabilized. Microbial Fe(II) oxidation and precipitation as schwertmannite together with aggregation of colloidal and nano-particulate Fe(III) are suspected to be the main removal mechanisms. Iron solubility limited removal under very acidic conditions (pH < 3). The reproducibility of the results with respect to previous research confirmed that VFRs can be used as stand-alone passive treatment systems for iron removal from mine waters with a footprint less than half of the area required by a conventional aerobic wetland. A VFR can also provide useful iron pretreatment for other passive treatment systems under circumneutral conditions, but would have to be combined with alkaline generating systems to achieve full iron removal from acidic mine waters.

     

Enhancement of Sludge Sedimentation Properties in a Concentrated Acid Mine Drainage Using Nano- and Micro-magnetite Particles

Conventional treatment of AMD involves neutralization with consequent precipitation of metals as hydroxides. In AMD with a high concentration of metals, the settling rate of the sludge/water interface is low. We investigated the use of nano- and micro-magnetite particles to assist the settling and thickening of floc particles. The magnetite was produced from ferrous sulphate crystals (melanterite, Fe₂SO₄·7H₂O) obtained by leaching pyrite from a coal mine. AMD was obtained from the treatment plant at the same mine and the water was neutralized with Ca(OH)₂ at pH 8.7?±?0.1. Laboratory studies were conducted in 1 L test tubes with and without the addition of magnetite particles and a flocculant. Sedimentation curves (interface settling) were generated to evaluate the rate of sedimentation. For the studied effluent, the best option was 4 g L^?1 of magnetite particles and 5 mg L^?1 of high molecular weight anionic polyacrylamide. The magnetite particles were recovered magnetically from the sludge with ≈ 90% efficiency. Thus, the combined use of magnetite and a flocculant increased the sludge settling rate and, consequently, reduced the area needed for settling basins.